1. Field of the Invention
The present invention relates to an arc detector for a plasma processing system employed in a semiconductor manufacturing process. The present invention further relates to a program for operating a computer as an arc detector, and also to a recording medium for storing such a program.
2. Description of the Related Art
FIG. 7 is a schematic block diagram showing a basic structure of a conventional plasma processing system. The system includes a high frequency power source 51, an impedance matching unit 53, and a plasma processing unit 55. The high frequency power source 51 and the impedance matching unit 53 are connected via a transmission cable 52, and the impedance matching unit 53 and the plasma processing unit 55 are connected via a load connection unit 54. The impedance matching unit 53 serves to match the impedance of the high frequency power source 51 (input impedance) and that of the plasma processing unit 55 (load impedance).
The high frequency power source 51 includes a power amplifying circuit, an oscillating circuit, and so forth. The high frequency power source 51 outputs a high frequency power of a predetermined value to the plasma processing unit 55, via the impedance matching unit 53.
The plasma processing unit 55 is an apparatus used to process a workpiece such as a semiconductor wafer or a liquid crystal substrate, by etching, CVD or the like. Specifically, the plasma processing unit 55 includes a plasma chamber, a vacuum container provided in the chamber, and a pair of electrodes provided in the vacuum container. Into the vacuum container a gas for generating plasma (such as nitrogen gas or argon gas) is introduced, and the high frequency power is supplied between the pair of electrodes. As a result, the gas is ionized thereby turning into plasma, which performs the processing of the workpiece.
In the plasma processing unit 55, an arc may be generated during the plasma processing. The arc thus generated may damage the workpiece, or even destroy the plasma processing unit 55 in case the arc is excessively intense. Generally, therefore, the high frequency power source 51 is provided with a function of detecting the arc generation, so as to reduce the high frequency power to be supplied to the plasma processing unit 55, or to completely cut off the power supply.
For example, JP-A-H08-167500 discloses the following system. Firstly a forward power into the plasma processing unit and a reflected power from the plasma processing unit are detected. According to a fluctuation pattern of the detected values, it is determined whether an arc has been generated. Upon deciding that an arc has been generated, the output of the high frequency power source is restrained, or even suspended as the case may be.
In the plasma processing unit 55, as already stated, the high frequency power is applied to the nitrogen gas or argon gas enclosed in the vacuum container to thereby turn the gas into plasma, and to perform a predetermined plasma processing. During such operation, the impedance of the plasma processing unit 55 is constantly fluctuating, and hence unstable. Accordingly, the impedance matching unit 53 serves to facilitate the high frequency power source 51 to efficiently supply the high frequency power to the plasma processing unit 55, under the situation where the impedance of the plasma processing unit 55 is fluctuating.
While the plasma processing unit 55 is normally performing the plasma processing, the impedance matching unit 53 effectively serves to match the impedance, and hence the reflected power from the plasma processing unit 55 is quite small. Once an arc is generated in the plasma processing unit 55, however, the impedance matching process may be disturbed. Generation of an arc that can, for example, destroy the workpiece or the plasma processing unit 55 with a single discharge action (hereinafter, “hard arc”) causes the impedance of the plasma processing unit 55 to fluctuate so sharply, that the impedance matching unit 53 can no longer track the impedance fluctuation. In this case, therefore, the reflected power from the plasma processing unit 55 is sharply increased.
The arc detection system according to JP-A-H08-167500 monitors the fluctuation rate of the reflected power from the plasma processing unit, and determines that an arc discharge has occurred when the fluctuation rate exceeds a predetermined threshold value. Such conventional detection method primarily aims at detecting a hard arc, and hence a relatively high threshold value is set.
On the other hand, some arcs are so weak (hence called “soft arcs”) that a single discharge does not damage the workpiece, but a succession of them can incur an uneven processing effect or a burn on the workpiece. These days the generation of the soft arc during the plasma processing has also come to be focused on, in order to maximize the quality of the workpiece.
Based on the conventional arc detection method, it apparently seems that lowering the threshold value for detecting the arc generation enables detecting the soft arc. However, the generation of the soft arc barely causes the fluctuation of the reflected power. This makes it quite difficult to distinguish whether a detected fluctuation rate of the reflected power is resultant from the soft arc generation, or from the natural impedance fluctuation. Lowering the threshold value under such circumstance often provokes erroneous detection of the arc generation, from what is actually the natural impedance fluctuation. Moreover, restraining or cutting off the supply of the high frequency power to the plasma processing unit based on the erroneous detection of the arc generation leads to significant degradation of the productivity of the liquid crystal substrate and other objects.